The morphology of particles of grafted polyolefins is dependent on the polymerization conditions and on the porosity of the material used as the backbone of the graft copolymer. When the porosity of the starting material is too low, a typical polypropylene graft copolymer with 85 parts of monomer added per hundred parts of polypropylene has a tendency to form a surface layer with a high polymerized monomer content ("shelling"). When the monomer add level is high, this shelling often produces a tacky surface on the particles, resulting in poor flowability of the polymer particles, which in turn may cause reactor fouling.
A variety of polymerization inhibitors have been used during graft polymerization reactions. For example, U.S. Pat. No. 3,839,172 discloses a process for the radiation grafting of acrylic monomers onto perhalogenated olefin polymer substrates in which a polymerization inhibitor such as ferrous ammonium sulfate or copper chloride is present in the grafting medium to prevent homopolymerization of the acrylic monomer. U.S. Pat. No. 4,196,095 discloses the use of a polymerization inhibitor such as a combination of copper and copper acetate in a process for the radiation grafting of a hydrophilic compound onto a hydrophobic substrate in the presence of a crosslinking agent and a polar solvent-soluble substance. U.S. Pat. No. 4,377,010 discloses the use of homopolymerization inhibitors such as ferrous sulfate or potassium ferricyanide during the radiation-initiated graft polymerization of acrylic monomers onto a base polymer to make a biocompatible surgical device. U.S. Pat. No. 5,283,287 discloses the use of polymerization inhibitors such as catechol, hydroquinones, organic sulfides, and dithiocarbamates to control the sequence of acrylonitirile units in a process for preparing thermoplastic resin compositions having excellent HCFC resistance.
However, there is still a need for polymerization rate modifiers that will inhibit surface polymerization during the graft polymerization of propylene polymer materials and therefore improve the processability of the resulting polymer particles.